Abstract:
Fungi have evolved a remarkable diversity of reproductive strategies. Some of these, most notably those of the model fungi,
have been well studied but others are poorly understood. The latter is also true for uni-directional mating type switching,
which has been reported in only five fungal genera, including Ceratocystis. Mating type switching allows a self-fertile fungal
isolate to produce both self-fertile and self-sterile offspring. This study considered the molecular nature of uni-directional
mating type switching in the type species of Ceratocystis, C. fimbriata. To do this, the genome of C. fimbriata was first
examined for the presence of mating type genes. Three mating genes (MAT1-1-1, MAT1-2-1 and MAT1-1-2) were found in an
atypical organisation of the mating type locus. To study the effect that uni-directional switching has on this locus, several
self-sterile offspring were analysed. Using a combination of next generation and conventional Sanger sequencing, it was
shown that a 3581 base pair (bp) region had been completely deleted from the MAT locus. This deletion, which includes the
entire MAT1-2-1 gene, results in the permanent loss of self-fertility, rendering these isolates exclusively self-sterile. Our data
also suggest that the deletion mechanism is tightly controlled and that it always occurs at the same genomic position. Two
260 bp direct repeats flanking the deleted region are strongly implicated in the process, although the exact mechanism
behind the switching remains unclear.
